1. Field of the Invention
The present invention related to an information recording medium for recording and regenerating informations therein such as magneto-optical disk and optical disk.
2. Description of Related Art
In order to stably regenerate information for a long time in an information recording medium, it is necessary to stably keep the information. However, if an information recording medium using a hygroscopic substrate, such as a plastic substrate, is placed under a high-humidity condition, the substrate is hygroscopically swollen by the hygroscopic action thereof to be tilted as a whole, i.e. warped, or thereby leading to errors and the like in the recording and regeneration, and thus the reliability of information is missing. Accordingly, such an information recording medium has been provided with a protective layer having the humidity resistance, i.e., little or no water absorptivity in order to prevent the hygroscopic behavior of the substrate.
FIG. 1 is a sectional view showing principle parts of the conventional magneto-optical disk belonging to the same kind as that disclosed in, for example, Japanese Patent Application Laid-Open No. 60-115037 (1985). Referring to FIG. 1, a first protective layer 2 made of SiNx is formed on one side of a light-transmitting plastic substrate 1 made of polycarbonate resin. A recording layer 3 made of a TbFeCo magnetic layer is formed on the first protective layer 2. A second protective layer 4 made of the same material as that of the first protective layer 2 is formed on the recording layer 3. A light-transmitting third protective layer 5 made of ultraviolet ray-setting resin is formed on the other side of the plastic substrate 1. A laminated member comprising the plastic substrate 1, the first-, second- and third protective layers 2, 4, 5 and the recording layer 3 is provided with a humidity resisting sealing member 6 made of ultraviolet ray-setting resin at an end portion thereof.
With such a magneto-optical disk, the first-, second- and third protective layers 2, 4, 5 and the humidity resisting sealing member 6 prevent moisture from entering the plastic substrate 1, thereby eliminating the hygroscopic action of the plastic substrate 1.
The conventional magneto-optical disk having the above described construction is provided with the respective protective layers 2, 4, 5 and the like but it is difficult to reduce the moisture permeability thereof to zero, so that the plastic substrate 1 is unavoidably hygroscopic to be swollen under a high-humidity condition. On the contrary, problems have occurred in that the respective protective layers 2, 4, 5 do not readily expand, whereby the magneto-optical disk is tilted as a whole (warped) due to differences between the respective layers in expansion coefficient or swelling rate.
In addition, problems have occurred also in that once moisture is absorbed by the plastic substrate 1, the moisture is kept in the plastic substrate 1 for a long time in the opposite way due to the respective protective layers 2, 4, 5, whereby not only a time, when the magneto-optical disk is tilted as a whole, is increased but also the recording layer 3 is oxidized and corroded.